Methods for the investigation of thyroid function

In the developed world, all routine clinical laboratories should be able to perform tests for the assessment of the pituitary-thyroid axis. Testing strategies usually involve the measurement of thyroid-stimulating hormone (TSH), either alone or in combination with free thyroxine (FT4), which itself should also be measured when TSH is abnormal or if there is a suspicion of pituitary disease. Based on these findings, clinical history and medications such as amiodarone, free tri-iodothyronine (FT3), thyroid-binding globulin (TBG), and/or autoantibodies may then be measured. All these assays have their individual challenges. For example, sensitivity of a TSH assay is of crucial importance for improving distinction between eu- and hyperthyroidism. In the measurement of free hormones, there is the potential disturbance of the equilibrium between free and bound moieties by separating systems. There are a number of commercial diagnostic kits available for each of these tests. This chapter presents various representative assays and their principles. With the exception of FT4 determination by equilibrium dialysis, those that employ a 96-well plate format and use non-isotopic techniques have been described. Attention has been paid to limitations of the assays, standardization of assay kits, imprecision, recovery, detection limit, and ease of use. The assays described have been given as typical and reliable examples, but it is the responsibility of assayists to choose methods that are best suited to their needs.